1. Field of the Invention
This invention relates to high strength, low modulus biocompatible medical implants made from titanium-niobium-zirconium alloys. In particular, implants of the invention have a modulus of elasticity closer to that of bone than other typically-used metal alloys and do not include any elements which have been shown or suggested as having short term or long term potential adverse effect.
2. Background of the Invention
The most common materials used for load-bearing medical implants such as hip or bone prostheses are metallic alloys, ceramics and composites formed of biocompatible polymers and various reinforcing materials.
Polymers are typically used in implants such as intraocular lenses, facial bone remodeling and other non-load bearing applications. In order to use plastic materials in load-bearing applications, they are typically reinforced with a particulate or high modulus fibrous material, such as carbon fiber, to produce composites of acceptable strength capable of withstanding relatively great applied loads. Although composites are presently under consideration by many companies, their usefulness as an implant material lies in their relatively low elastic modulus compared to metal and ceramic implants, and their optimum design characteristics are still being explored.
Ceramic prostheses provide excellent biocompatibility, corrosion resistance (inertness) and high compression strength for load-bearing applications. However, ceramic prostheses are typically rigid (high elastic modulus), and unyielding under stress from loads applied during normal use, so that it cannot effectively transfer stresses to surrounding bone. Thus, bone decalcification which results in localized thinning (resorption) and weakening of the bone structure may occur.
Metals and metal alloys such as stainless steel, vitalium (cobalt alloy) and titanium have been used successfully. These materials have the requisite strength characteristics but typically have not been resilient or flexible enough to form an optimum implant material. Also, many alloys contain elements such as aluminum, vanadium, cobalt, nickel, molybdenum, and chromium which recent studies have suggested might have some long term adverse effects on human patients.
Many of the metal alloys typically used in prosthetic implants were developed for other applications such as Ti-6A1-4V in alloy in the aircraft industry. These alloys were later thought to be suitable for use as implant materials because they possess mechanical strength and appeared to have acceptable levels of biocompatibility. However, these metals typically have elastic moduli much higher than that of bone, for example, 316 stainless steel has an elastic modulus of about 200 GPa while that of cast heat-treated Co--Cr--Mo alloy is about 240 GPa. 0f these, the alloy with the lowest elastic modulus is Ti-6A1-4V with an elastic modulus of about 120 GPa.
It has also been found that many of these metals will corrode to some extent in body fluids thereby releasing ions that might possibly be harmful over a prolonged period of time. It is now believed that the corrosive effects of body fluids is due both to chemical and electro-chemical processes, with corrosion products forming when certain commonly-used metal alloys ionize from corrosion processes in the body. For example, aluminum metal ions have been associated with Alzheimer's disease and vanadium, cobalt, molybdenum, nickel and chromium are suspected of being toxic or carcinogenic.
It has been suggested that metals could be coated with a biocompatible plastic, ceramic or oxide to overcome the corrosion problem. However, coatings tend to wear off, especially on articulating bearing surfaces of total joints, and are susceptible to galling and separating from the metal substrate, exposing the metal to body fluids.
Generally, it is the industry practice to passivate the implant metal alloys. However, passivation produces only thin amorphous, poorly attached protective oxide films which have not proved totally effective in eliminating the formation of corrosion products in the body, particularly in situations where fretting occurs in the body.
Titanium alloys offer advantages over the stainless steels because of their lower susceptibility to corrosion in the body coupled with their high strength and relatively low modulus of elasticity. Upon cooling, the currently used Ti-6A1-4V alloy transforms from a .beta.-structure to an .alpha.- plus .beta.-structure at about 1000.degree. C. This transition can be shifted to a lower temperature by the addition of one or more suitable .beta.-phase stabilizers such as molybdenum, zirconium, niobium, vanadium, tantalum, cobalt, chromium, iron, manganese and nickel.
Some efforts have been directed toward the development of alloys that eliminate harmful metals. For example, U.S. Pat. No. 4,040,129 to Steinemann et al. is directed to an alloy which includes titanium or zirconium as one component and, as a second component, any one or more of: nickel, tantalum, chromium, molybdenum or aluminum, but does not recognize or suggest any advantages from having a relatively low elastic modulus, or advantages or disadvantages associated with high temperature sintering treatments (at about 1250.degree. C.), commonly employed to attach porous metal coatings into which bone can grow to stabilize non-cemented, press-fit devices into the skeletal structure. Steinemann also indicates that the ultimate tensile strength should be greater than about 979 MPa (142 ksi) with a minimum tensile elongation of 10%.
Although Steinemann provides that copper, cobalt, nickel, vanadium and tin should be excluded, apart from their presence as unavoidable impurities, the patent indicates that it is permissible to have any or all of chromium, molybdenum and aluminum, which are all believed to have potential long-term adverse effects, present in the alloy as long as their combined weight does not exceed 20% of the total weight of the alloy.
U.S. Pat. No. 4,857,269 to Wang et al. is not a statutory bar and its citation is not an admission that its teachings are applicable prior art. This patent relates to a titanium alloy for a prosthetic implant said to have high strength and a low modulus. The titanium alloy contains up to 24 wt. % of at least one isomorphous beta stabilizer from the group molybdenum, tantalum, zirconium and niobium; up to 3 wt. % of at least one eutectoid beta stabilizer from the group iron, manganese, chromium, cobalt or nickel; and optionally up to 3 wt. % of a metallic .alpha.-stabilizer from the group aluminum and lanthanum. Incidental impurities up to 0.05% carbon, 0.30% oxygen, 0.02% nitrogen, and up to 0.02% of the eutectoid former hydrogen are also included. Although there is some discussion of having an elastic modulus (e.g., Young's modulus) around 85 GPa, the only examples of a low modulus (66.9-77.9 GPa) all contain 11.5 wt. % Mo which is a potentially toxic element and undesirable for optimizing biocompatibility.
Other currently used metal alloys have similar drawbacks. For example, the commonly used Ti-6A1-4V alloy, with appropriate heat treatment, offers some degree of biocompatibility but has an elastic modulus of about 120 GPa. Although this elastic modulus is lower than other alloys and accordingly offers better load transfer to the surrounding bone, this modulus is still significantly greater than desired. Moreover, the alloy contains aluminum and also vanadium, which is now suspected to be a toxic or carcinogenic material when present in sufficient quantity.
Commercially available PROTOSUL 100 (Sulzer Bros. Ltd.) is a Ti-6Al-7Nb alloy which intentionally avoids the potentially adverse effects of vanadium toxicity by substituting niobium. However, the alloy still contains aluminum and has an elastic modulus of about 110 GPa (15.9.times.10 psi) in heat-treated condition, and with a tensile strength of about 1060 MPa.
With orthopedic prostheses being implanted in younger people and remaining in the human body for longer periods of time, there is a need for an implant material with requisite strength and flexibility requirements, which does not contain elements which are suspected as having long-term harmful effects on the human body.